1. Field of the Invention
The present invention relates to a motor and a motor manufacturing apparatus, in particular, to cogging torque reduction.
2. Description of the Related Art
In motors comprising a rotor with a magnet embedded therein and a stator having slots provided equidistantly in a circumference and a coil portion formed therein by winding in the slots, those motors being rotationally driven by the torque of the magnet and the reluctance torque of the coil, when the rotor rotates, a pulsating cogging torque is generated in the rotor.
When the magnet disposed inside the rotor forms a magnetic circuit in which the generated magnetic flux is closed via the slots in the stator, the total sum of electromagnetic attraction forces at a rotation angle of the rotor with respect to the stator in the rotation direction varies according to the relative position of the rotor and stator. Because the stator is composed of repeating slots and slot open portions having different magnetic resistances, the electromagnetic attraction force switches in the rotor in the rotation direction or counter-rotation direction of the rotor when one slot pitch is passed as the rotor rotates, the torque varies with a period proportional to the number of poles in the magnet or the number of slots, and a cogging torque is generated.
Conventional measures aimed at the reduction of this cogging torque included the improvement of rotor shape, stator shape, and accuracy of dies used for forming the rotor and stator. For example, skewing is a known method by which when the rotor is stacked in the axial direction, it is skewed by arranging with an equidistant displacement in a circumference direction which is the rotor rotation direction. With such a method, when the magnetic poles of the magnets disposed in the rotor pass by the slots of the stator, the relative positions are shifted in a circumferential direction, thereby causing the positions with a maximum clogging torque generated in each layer to shift in the circumference direction. Furthermore, multistep skewing with division into no less than two steps in the axial direction is also known.
Despite such a reduction of cogging torque based on the rotor shape and stator shape, the cogging torque still remains. The inventors have discovered that magnetic anisotropy of flat rolled magnetic steel sheets and strip constituting the rotor and stator causes the cogging torque. Rotor cores and stator cores used in motors are manufactured from non-directional flat rolled magnetic steel sheets and strip, but even in the non-directional flat rolled magnetic steel sheets and strip magnetization characteristics differ in the direction parallel to the rolling direction and the direction perpendicular to the rolling direction. The inventors have discovered that magnetic anisotropy of flat rolled magnetic steel sheets and strip becomes the cause of the cogging torque.
In Japanese Patent Application Laid-open No. 10-66283 (hereinafter referred to as Patent Document 1), the inventors have suggested measures for eliminating rotation fluctuations caused by magnetic anisotropy in synchronous electric motors. Patent Document 1 is focused on magnetic anisotropy of stacked stators and discloses a method for the manufacture of a stacked stator which reduces torque ripple caused by the magnetic anisotropy in synchronous electric motors.
However, the rotation fluctuations which are the object of elimination in Patent Document 1 represent the torque ripple and are different from the cogging torque. The inventors, to the best of their knowledge, have no references that have to be disclosed with respect to prior art technology aimed at the reduction of cogging torque caused by magnetic anisotropy of flat rolled magnetic steel sheets and strip.
When a core workpiece of flat rolled magnetic steel sheets and strip is punched out with a die and a plurality of cores are stacked by lamination, because the core shapes are arranged in the same direction with respect to the rolling direction, magnetic anisotropy of the core causes the cogging torque. The torque ripple which is the object of elimination in Patent Document 1 represents an effect appearing when an electric current is passed in the motor. By contrast, the cogging torque which is to be eliminated in accordance with the present invention is not related to the electric current flowing in the coil, is determined by the shape of the core or the magnet, and is caused by different factors.
Therefore, the task of eliminating the cogging torque caused by magnetic anisotropy of a motor has not been disclosed in Patent Document 1 and, to the best of the inventors' knowledge, is set for the first time in the present invention.
Methods of skewing or step shifting that have been conventionally used for reducing the cogging torque are designed to reduce the cogging torque caused by the shape factors such as the rotor shape or stator shape. The cogging torque generated by the shape factors has a comparatively large amplitude and the methods comprising skewing or step shifting are effective for reducing such a cogging torque having a large amplitude. However, the cogging torque component caused by magnetic anisotropy is impossible to reduce by such methods.
Furthermore, stacking the cores while rotating them so as to obtain an almost uniform arrangement in the entire stacked core at a pitch of poles, slots, or outer shape can be also considered as means for averaging the scattering of the core thickness or the scattering caused by the core shape associated with die accuracy, but such a uniform arrangement cannot reduce the cogging torque caused by magnetic anisotropy.